Some grammarians (such as Sibawayh) argue that it is an abbreviation of يا الله أمّ بخير (yā allāhu ʾumm bi-khayr) (with the meaning of "O God, bestow (on us) goodness"); others have argued without explanation that the suffix ـ هُمَّ (-humma) takes the place of yā (O).
see also