allyyoudaoicibaDictYouDict[ally 词源字典]
ally: [13] The verb ally was borrowed into English from Old French alier, an alteration of aleier (a different development of the Old French word was aloier, which English acquired as alloy). This came from Latin alligāre ‘bind one thing to another’, a derivative of ligāre ‘tie’; hence the idea etymologically contained in being ‘allied’ is of having a bond with somebody else.

The noun ally seems originally to have been independently borrowed from Old French allié in the 14th century, with the meaning ‘relative’. The more common modern sense, ‘allied person or country’, appeared in the 15th century, and is probably a direct derivative of the English verb.
=> alloy, ligament[ally etymology, ally origin, 英语词源]
ally (v.)youdaoicibaDictYouDict
late 13c., "to join in marriage," from Old French alier "combine, unite," from a differentiated stem of aliier (from Latin alligare "bind to;" see alloy). Meaning "to form an alliance, join, associate" is late 14c. Related: allied; allying.
ally (n.)youdaoicibaDictYouDict
late 14c., "relative, kinsman," from ally (v.); mid-15c. in the sense of "one united with another by treaty or league."