typical sloppy American usage, Hey--we're not sloppy; we're just practical.

Helen--I put precip* into the Onelook search box, and was astonished to find 95 listings! One sort-of clue came up as I went fishing around. I looked up precipe in M-W, and found it is a variant of PRAECIPE:
Main Entry: praeˇciˇpe
Pronunciation: 'pre-s&-"pE, 'prE-
Function: noun
Etymology: Middle English presepe, from Medieval Latin praecipe, from Latin, imperative of praecipere to instruct -- more at PRECEPT
1 : any of various legal writs commanding a person to do something or to appear and show cause why he or she should not
2 : a written order requesting a clerk or prothonotary of a court to issue a writ and specifying the contents of the writ So then I looked up precept and found the clue, which I'll ...um... embolden.

Main Entry: preˇcept
Pronunciation: 'prE-"sept
Function: noun
Etymology: Middle English, from Latin praeceptum, from neuter of praeceptus, past participle of praecipere to take beforehand, instruct, from prae- + capere to take -- more at HEAVE
1 : a command or principle intended especially as a general rule of action
2 : an order issued by legally constituted authority to a subordinate official
synonym see LAW
I'd been thinking that in precipitate, the pre wasn't exactly a real "pre"fix, but. Searching for cipitate yielded no results found.