The phrase "hadst dibs something" is an archaic way of saying "claimed something". To find synonyms for this phrase, we can look at words that mean to take possession or assert ownership over something. Some possible synonyms include: laid claim to, staked a claim, commandeered, grabbed, seized, appropriated, acquired, requisitioned, reserved, or secured. All of these words convey the idea of asserting ownership over something, whether it's a physical object or a more abstract concept. While "hadst dibs" may not be a commonly used phrase today, there are plenty of other words that can be used to convey the same idea.