"slight sin," 1590s (earlier in corrupt form peccadilian, 1520s), from Spanish pecadillo, diminutive of pecado "a sin," from Latin peccatum "a sin, fault, error," noun use of neuter past participle of peccare "to miss, mistake, make a mistake, do amiss; transgress, offend, be licentious, sin," a word of uncertain origin.
Watkins traces it to PIE *ped-ko-, suffixed form of *ped- "to walk, stumble, impair," from root *ped- "foot." But de Vaan is suspicious: "there is no reference to feet in the meaning of peccare. And to 'make a faux pas' ... would hardly be rendered by the word for 'foot', but rather by 'walking.' " He finds a derivation from the root *pet- "to fall" via *pet-ko- "a fall, error" to be "better semantically, but the addition of *-ko- to the bare root seems strange."