noun
lover of a married woman; adulterous lover
Refers specifically to a man who is the lover of a married woman; carries a secretive, illicit nuance. Not a general term for 'boyfriend' or 'lover'.
彼女には密夫がいるらしい。
It seems she has a lover on the side.
密夫との関係が夫にばれてしまった。
Her relationship with her lover was discovered by her husband.
愛人 is a broader term for a lover or mistress, not necessarily secret or adulterous, while 密夫 specifically denotes a secret male lover of a married woman.
不倫相手 is a more direct and common term for an affair partner, applicable to any gender, whereas 密夫 is a less common, somewhat literary word specifically for the male lover.
Compound of 密 (secret) and 夫 (husband/man), literally 'secret man'. The exact historical derivation is uncertain, but the kanji clearly convey the meaning of a clandestine male lover.