The meaning “to restrain, to withhold,” also rather prominently attested in Arabic, opens an interesting possibility of comparison with PWS *ḥkm; otherwise with no etymology (Kogan 2015:82).
Variants: libbuḫu, ilibbuḫu, ellambuḫu, ilbuḫu, illabuḫu, ellabuḫu. Von Soden suggests that this is a Sum. loanword (see a detailed discussions in [Lieberman 222]). Part of the forms with prefixed ˀi- (< *ˀa- influenced by -i- of the second syllable implying *ˀa-libbuḫ- ?).