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blot (one's) copy book

To tarnish, damage, or ruin one's reputation by behaving badly or committing some mistake or social transgression. Refers to a child's copy book, the blotting (staining with ink) of which ruins one's work. Primarily heard in UK. The local councilor blotted his copy book when it came to light that he had accepted bribes to allow unregulated development projects to go ahead. I really blotted my copy book when I spilled my drink on the visiting dignitary last night.
See also: blot, book, copy
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

blot your copybook

If you blot your copybook, you damage your reputation by doing something wrong. Their relationship had been so perfect. Until he'd blotted his copybook with Susan, that is. Note: You can also say that there is a blot on your copybook. In fact, just about the only blot on his copybook so far was a missed penalty against Arsenal 10 days ago. Note: In the past, schoolchildren had `copybooks'. These were books of examples of handwriting, with spaces for the children to copy it.
See also: blot, copybook
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

blot your copybook

tarnish your good reputation. British
A copybook was an exercise book with examples of handwriting for children to copy as they practised their own writing.
See also: blot, copybook
Farlex Partner Idioms Dictionary © Farlex 2017

blot your ˈcopybook

(old-fashioned, informal) spoil a previously good record: He paid back the money he had stolen, but he had blotted his copybook and couldn’t hope for promotion.A copybook was an exercise book with printed examples of good writing for children to copy. If you blotted it, you accidentally spoiled it by making a mark with ink on it.
See also: blot, copybook
Farlex Partner Idioms Dictionary © Farlex 2017
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References in periodicals archive ?
Showing the copybook to the jury, Det Gda Daly agreed with Mr Grehan it contained a "drawing of a satanic pledge" with star-shaped symbols in one corner.
* Working-Storage Section - allows entry of up to eight (8) copybook names and will expand uploaded copybooks for Working-Storage
Sharps impressively marshalled a strong defensive display from the hosts but blotted his copybook by being shown a straight red card for hauling down Gillingham's top scorer Cody McDonald.
The event is on Friday, February 11, and is a must for darts enthusiasts with top class action as Painter's opponents will include a host of talent and characters, randomly chosen from audience, who will be out to blot the copybook of the The Artist.
Wawrinka had a chance to blot the defending champion's copybook when he went a break up in the second but Federer predictably hit back and secured the set on a tiebreak.
But Paul McLaughlin blotted his side's copybook in 75 minutes when he kicked Frew keeper Iain Wilson during a melee in the box and was sent off.
The Salford-based player scored 15 goals for the Seagulls last season and won the Supporters' player of the year, but two red cards in the last four games blotted his copybook.
Harry flew a Firefly training plane for about 30 minutes before making a copybook landing at the RAF Barkston strip.
Not a single red mark blotted the copybook for the market's sub-indices, as materials and consumer staples did not move and all other sectors found greener pastures.
The former Arsenal and Manchester City player has blotted his copybook in Turkey after falling out with a succession of clubs.
Designed to correspond with "A Greek Alphabetarion", A Greek Hupogrammon: A Beginner's Copybook for the Greek Alphabet with Pronunciations is a self-instructional, consumable workbook for anyone age 8 to adult who is beginning studies in reading, writing, and speaking the Greek language.
Formula One's most experienced and successful driver Michael Schumacher blotted his copybook at Silverstone yesterday with a mistake more familiar to fans in the grandstands than a seven-time world champion.
The solitary untruth which blotted Walter Smith's copybook?
The works appear to be part of a copybook as at least two of the portraits are adaptations of already published images.
The reader could draw a line down the middle of a copybook and entitle one side "Popular Perception" and the other "How It Was" and come up with an excellent summary of the book.