for a/some reason/reasons best known to himself, herself, etc.

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