noun
close relationship; intimate bond; thick as thieves
Colloquial and somewhat rare; describes a very close, often romantic or deeply affectionate relationship. The word 濃い (thick, dense) metaphorically suggests a strong, inseparable bond. Compare with 恋仲 (こいなか), which specifically means 'in love with each other'.
See also: 恋仲
あの二人は濃い仲らしいよ。
Those two seem to be really close.
彼とはただの友達じゃなくて、濃い仲なんだ。
He and I aren't just friends; we have a deep bond.