close friend; bosom buddy; pal
Playful, emphatic term for a very close friend; often used in casual, friendly contexts. The repetition of よし adds a lighthearted, rhyming quality.
See also: 仲良し
He and I have been bosom buddies since we were kids.
Those two are always together; they're really close pals, aren't they?
仲良し is the standard, neutral term for a close friend. 仲良しこよし is a more playful, emphatic variant with a rhyming repetition.
Standard spelling; the repetition of よし gives a playful, emphatic feel.
Rarely used kanji form.
Derived from 仲良し (close friend) with the playful, rhyming repetition of よし, similar to other emphatic reduplicative expressions in Japanese.