dip (one's) pen in gall

To write something that conveys one's animosity, anger, or malice. The critic must have dipped his pen in gall before writing that very negative review. Hey, don't dip your pen in gall and dash off an angry message. Maybe cool off a little bit first. Well, that was a brutal performance review. The boss certainly dipped his pen in gall before filling it out.

dip your pen in gall

write unpleasantly or spitefully.

Gall is another word for bile, the bitter secretion of the liver; it is used in many places in the Bible as a metaphor for bitterness or affliction. See also wormwood and gall (at wormwood).