have a handle on

Old English handle, formed from hand (n.) with instrumental suffix -le indicating a tool in the way thimble was formed from thumb. The slang sense of "nickname" is first recorded 1870, originally U.S., from earlier expressions about adding a handle to (one's) name, i.e. a title such as Mister or Sir, attested from 1833. 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 first recorded 1972.