Indecent means offending against accepted standards, usually of modesty. Indecent exposure, indecent images, indecent behaviour.
In law it is a real category: indecent exposure and indecent assault are offences, though the wording has been modernised in many places. Outside law it is milder and often ironic — an indecent amount of cake. Indecent haste is the fixed phrase for moving faster than looks decent, as when somebody remarries or resigns too quickly. Note indecency; and decent, which is far commoner in ordinary speech.
1adjectiveindecentOffending against accepted standards of modesty.
“The magazine was banned for printing indecent photographs.”
2adjectiveimproperlyImproperly quick or excessive.
“She remarried with indecent haste, just a month after her husband's funeral.”
indecent is a quality of impropriety.
In- plus decent, from Latin decere, to be fitting. Decorum, decorate and dignity come from the same root, so indecent is literally unfitting.
Proto-Indo-European
*deḱ- — to take, accept, to receive, greet, be suitable
Latin
decēns
Middle French
décent
English
decent
English, today
indecent
Built from
From in + decent. decent Middle French décent, or its source, Latin decēns, present participle of decet (“it is fitting or suitable”), from Proto-Indo-European deḱ- (“to take, accept, to receive, greet, be suitable”) (compare Ancient Greek δοκέω (“to appear, seem, think”), δέχομαι (“to accept”); Sanskrit दशस्यति (“shows honor, is gracious”), दाशति (“makes offerings, bestows”)). Meaning ‘kind, pleasant’ is from 1902.
Etymology from Wiktionary, CC BY-SA