to do for; to do a favor (casual)
Expresses doing something as a favor for someone else, often with a casual or even condescending tone.
てやる means to do something for someone, similar to てあげる but more casual and often used when the speaker is in a higher position or when the recipient is a pet, a child, or a close friend. It can sound rude if used toward someone of higher status, so avoid it in formal situations. In casual speech with friends, it can show willingness to help, but it may also imply 'I'll do this for you (since you can't)'. Use てあげる for polite favors, and てもらう/てくれる for when someone does something for you.