For a reason best known to himself - Idioms by The Free Dictionary
for a/some reason/reasons best known to himself, herself, etc. (redirected from for a reason 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.
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
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.