Etymology
Advertisement
ally (n.)
late 14c., "relative, kinsman" (a sense now obsolete), from ally (v.); mid-15c. in the sense of "one united with another by treaty or league." Allies as the name of the nations aligned against the Central Powers in World War I is from 1914; as the nations aligned against Germany, Italy and Japan in World War II, from 1939.
Related entries & more 
Advertisement
ally (v.)

late 13c., "to join in marriage" (transitive), from Old French alier "combine, unite," from a differentiated stem of aliier (from Latin alligare "bind to, tie to," from ad "to" (see ad-) + ligare "to bind, bind one thing to another, tie" (from PIE root *leig- "to tie, bind"). Meaning "to form an alliance, join, associate" is late 14c. Related: allied; allying.

Related entries & more 
re-ally (v.)

"to form an alliance again, to connect or unite again," c. 1600, from re- + ally (v.). A doublet of rally (v.1), with hyphenated spelling and full pronunciation of the prefix to distinguish it from really. Related: Re-allied; re-alliance.

Related entries & more 
allied (adj.)
c. 1300, past-participle adjective from ally (v.). Originally of kindred (compare alliance); in reference to a league or formal treaty, it is first recorded late 14c.
Related entries & more 
alliance (n.)
c. 1300, "bond of marriage" (between ruling houses or noble families), from Old French aliance (12c., Modern French alliance) "alliance, bond; marriage, union," from aliier (Modern French allier) "combine, unite" (see ally (v.)).

General sense of "combination for a common object" is from mid-14c., as are those of "bond or treaty between rulers or nations, contracted by treaty" and "aggregate of persons allied." Unlike its synonyms, "rarely used of a combination for evil" [Century Dictionary]. Meaning "state of being allied or connected" is from 1670s. The Latin word was alligantia.
Related entries & more 
Advertisement
*ad- 
Proto-Indo-European root meaning "to, near, at."

It forms all or part of: abate; ado; ad-; ad hoc; ad lib; adage; adagio; add; adjective; adore; adorn; adult; adverb; advertise; agree; aid; alloy; ally; amontillado; amount; assure; at; atone; exaggerate; paramount; rapport; twit.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit adhi "near;" Latin ad "to, toward;" Old English æt.
Related entries & more 
*leig- 

Proto-Indo-European root meaning "to tie, bind." 

It forms all or part of: alloy; ally; colligate; deligate; furl; league (n.1) "alliance;" legato; liable; liaison; lien; lictor; ligand; ligament; ligate; ligation; ligature; oblige; rally (v.1) "bring together;" religion; rely.

It is the hypothetical source of/evidence for its existence is provided by: Latin ligare "to bind;" Albanian lidh "I bind," and possibly Middle Low German lik "band," Middle High German geleich "joint, limb."

Related entries & more 
rally (v.1)

"bring together or into order again by urgent effort," c. 1600, from French rallier, from Old French ralier "reassemble, unite again," from re- "again" (see re-) + alier "unite" (see ally (v.)).

In Old French (and Italian), re- often appeared as ra- by confusion with the true ra- (from Latin re- + ad-), and the following consonant often was doubled; compare rabbet (a doublet of rebate), rappel (a doublet of repeal). But as ra- was not recognized in English as a prefix, words with ra- in Old French usually returned to re- in English; Rally and rabbet never were because the accent had receded. In later borrowings (rappel, rapprochement, etc.) the words tend to keep their French forms. 

Intransitive meaning "pull together hastily, recover order, revive, rouse" is from 1660s. Related: Rallied; rallying. Rallying-point "place at or about which persons come together for action" is by 1798. Rally round the flag (1862) is a line from popular American Civil War song "Battle Cry of Freedom."

Related entries & more 
alley (n.2)
also ally, type of large playing marble (generally one of stone as opposed to terra cotta), 1720, said to be a shortening of alabaster.
Related entries & more 
co-belligerent (n.)

"one who is mutually at war" (as distinguished from an ally), 1813, a word from the Napoleonic wars, from co- + belligerent. As an adjective, "carrying on war in conjunction with another power," from 1828.

Related entries & more