From Teflpedia
A man swearing an oath.

Swear is an English verb, originally meaning "to take an oath", and the derivational meaning "to use offensive language". The first use is original, but since taking an oath involves religious invocations, and inappropriate use of such religious invocations were considered taboo under Christianity, this developed into the meaning of "utter profanity".

Swear is an irregular verb - it has the third person form "swears", the -ing form "swearing", an irregular preterite "swore" and an irregular past participle "sworn". The last two may be over-regularised to *"sweared". Its pattern is similar to "bear", "tear", and "wear".

In the first usage this takes a to-infinitive, e.g. "I swear to tell the truth". It can also introduce a relative clause, e.g. "I swear that it wasn't me, officer!"