nepelkû, napalkû - to be wide (CAD N1 270, AHw. 778, EDA I 267)

Assuming that l is non-etymological, one can tentatively compare Gez. fakḥa “to be happy, to rejoice” (CDG 158), Amh. fäkka “to burst open, to bloom, to be ripe” (AED 2327), Har. fäkäḥa “to become bright, to bloom” (EDH 62), or, alternatively, Arb. fkh “to be cheerful, happy, free from straitness” (Lane 2432).