allayyoudaoicibaDictYouDict[allay 词源字典]
allay: [OE] In Old English, alecgan meant literally ‘lay aside’ (-a ‘away, aside, out’, lecgan ‘lay’). The more recent senses ‘relieve, mitigate’ developed from the 13th to the 15th centuries owing to the influence of two formally similar Old French verbs: aleger ‘lighten’ (from Latin alleviāre, source of English alleviate [15]); and al(e)ier ‘qualify, moderate’ (source of English alloy).
=> lay[allay etymology, allay origin, 英语词源]
allay (v.)youdaoicibaDictYouDict
Old English alecgan "to put down, remit, give up," a Germanic compound (cognates: Gothic uslagjan, Old High German irleccan, German erlegen), from a- "down, aside" + lecgan "to lay" (see lay).

Early Middle English pronunciations of -y- and -g- were not always distinct, and the word was confused in Middle English with various senses of Romanic-derived alloy and allege, especially the latter in an obsolete sense of "to lighten," from Latin ad- "to" + levis (see lever).
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.