Definition - the act or crime of knowingly making a false statement (as about a material matter) while under oath or bound by an affirmation or other officially prescribed declaration that what one says, writes, or claims is true
The Latinate prefix found at the beginning of perjury (per-) may mean a number of different things in English, such as "through," "throughout," or "containing the largest possible or a relatively large proportion of a (specified) chemical element especially as a result of exhaustive substitution for hydrogen or of exhaustive addition in an organic compound or group." However, in Latin per- could also mean "harmfully," and this is the one which is attached to perjury. If one commits perjury one is a perjurer, the adjectival form of the word is perjurious, and the adverb is perjuriously.
A man once in court wouldn't tell
the truth, and it didn't go well,
he spoke, so you see
he finds himself now in a cell