Latin phrase, literally "I think, therefore I am;" the starting point of Cartesian philosophy (see Cartesian), from cogito, first person singular present indicative active of cogitare "to think" (see cogitation) + ergo "therefore" (see ergo) + sum, first person singular present indicative of esse "to be" (from PIE root *es- "to be").
"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.)).