I heard Laith b. Said as saying: Al-Hamv means the brother of husband or like it from amongst the relatives of the husband, for example, cousin, etc.
سَمِعْتُ اللَّيْثَ بْنَ سَعْدٍ يَقُولُ الْحَمْوُ أَخُ الزَّوْجِ وَمَا أَشْبَهَهُ مِنْ أَقَارِبِ الزَّوْجِ ابْنُ الْعَمِّ وَنَحْوُهُ