Old English handle "a handle" (plural handla), formed from hand (n.) with instrumental suffix -el (1) indicating a tool in the way thimble was formed from thumb, spindle from spin, spindle from spin, ladle from lade, etc. The slang sense of "nickname" is first recorded 1870, originally U.S., from earlier expressions about adding a handle to (one's) name (1833), that is, a title such as Mister or Sir. To fly off the handle (1833) is a figurative reference to an ax head (to be off the handle "be excited" is recorded from 1825, American English). To get a handle on "get control of" is recorded by 1919.
Middle English hondlen, handlen, "touch with the hands, hold in the hands, fondle, pet," also "to deal with, treat, manhandle," from Old English handlian "to touch or move with the hands," also "deal with, discuss;" formed from hand (n.), perhaps with a frequentative suffix, as fondle from fond. Cognate with Old Norse höndla "to seize, capture," Danish handle "to trade, deal," Old High German hantalon "feel, touch; manage," German handeln "to bargain, trade." Related: Handled; handling. Meaning "to act towards" (someone, in a certain manner, usually with hostility or roughness) is from c. 1200. The commercial sense "to trade or deal in" was weaker in English than in some other Germanic languages, but it strengthened in American English (by 1888) from the notion of something passing through one's hands, and see handler.