comme il faut
"according to etiquette," 1756, French, literally "as it should be." From comme "as, like, how," from Old French com, from Vulgar Latin quomo, from Latin quomodo "how? in what way?," pronominal adverb of manner, related to quam "how much?," qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns). With il, from Latin ille "this" (see le) + faut, third person singular present indicative active of falloir "be necessary," literally "be wanting or lacking" (see fail (v.)).