"Welp" is an informal expression often used to convey resignation or acceptance of a situation, especially when it is unexpected or not ideal. It is similar to saying "Oh well," indicating that the speaker is acknowledging something unfortunate or disappointing but is ready to move on from it.
The word "welp" is likely a variation of the word "well," which has evolved in informal speech. It has roots in American English and became popular in the late 20th century, particularly in internet slang and memes.