for reasons best known to himself

for (a/some) reason(s) best known to (oneself)

For strange or perplexing reasons that the speaker cannot understand. For reasons best known to herself, Mom refuses to wear her glasses and goes around squinting at everything instead.
See also: known

for reasons best known to himself (or herself, etc.)

used when recounting someone's behaviour to suggest that it is puzzling or perverse. chiefly humorous
See also: himself, known, reason

for a/some reason/reasons best known to himˈself, herˈself, etc.

(humorous) for a reason or reasons which other people find hard to understand: For reasons best known to himself, he wears two pairs of socks.
See also: known, reason
References in classic literature ?
He was in his shirt, which was not long enough in front to cover his thighs completely and was six fingers shorter behind; his legs were very long and lean, covered with hair, and anything but clean; on his head he had a little greasy red cap that belonged to the host, round his left arm he had rolled the blanket of the bed, to which Sancho, for reasons best known to himself, owed a grudge, and in his right hand he held his unsheathed sword, with which he was slashing about on all sides, uttering exclamations as if he were actually fighting some giant: and the best of it was his eyes were not open, for he was fast asleep, and dreaming that he was doing battle with the giant.
And indeed, Father Brown, for reasons best known to himself, had lingered much longer than politeness required; or even, in the ordinary sense, permitted.
At uncertain times he suffers from acute attacks of the fidgets or cramp in his legs, and though he is more virtuous to behold than any of the cranes, who are all immensely respectable, he flies off into wild, cripple-stilt war-dances, half opening his wings and bobbing his bald head up and down; while for reasons best known to himself he is very careful to time his worst attacks with his nastiest remarks.
For reasons best known to himself, Martin saw this as a trap and thinks some sort of minority government would be preferable.
McAlorum created the final act of tragi-comedy when he scythed down Mouncey for reasons best known to himself.
For reasons best known to himself, "Worsted" seems to presume himself an average man, but fails to realise that today people have more leisure hours than ever before and would rather dress a little more casually - and comfortably - than merely "slipping off a jacket" for longer than a fortnight's holiday.
For reasons best known to himself, director Peter Rowe has chosen to set the story in 1950s Latin America, beginning with a colourful fiesta.
Felix Magath, for reasons best known to himself, is redesigning the Schalke model and I'm not convinced.
The first six spot-kicks were taken with tremendous poise until Kermorgant, for reasons best known to himself, attempted a cheeky chip from 12 yards, which was easily saved by Marshall.
ROBIN Hood is back fighting his corner against the Sheriff of Nottingham who for reasons best known to himself has taken up residence in Slaithwaite.
For reasons best known to himself, Harold eschewed the relative peace and quiet of the Crowne Plaza and instead bunked up with the hordes of Japanese and American tourists at the Beatles-themed Hard Day's Night Hotel.
Strangely, and for reasons best known to himself, he was wearing a high-visibility fluorescent jacket.
He was up against a lot of resistance to what he wanted to do and for reasons best known to himself he decided to move on.
For reasons best known to himself the author has adopted the American system of dates, with 'BCE' for BC and 'CE' for AD with no noticeably improvement to the quality of the story told.
For reasons best known to himself he decided that Port Talbot Town AFC would be his top UK team.