古英語の接頭辞a-「置く」「離れる」はPIE *uzに由来し、語源的にはoutと同じだが、ここでは接頭辞al-によって低俗化されている。 layは「置く」。
Amid the overlapping of meanings that thus arose, there was developed a perplexing network of uses of allay and allege, that belong entirely to no one of the original vbs., but combine the senses of two or more of them. [OED]The double -l- is 17c., a mistaken Latinism. Related: Allayed; allaying.