where it’s at
This slang expression gained widespread currency in the 1960s as a hip way of stating that the speaker understood the essential truth of a situation: “I know where it’s at.” Or more commonly, “You don’t know where it’s at.” It is still heard from time to time with that meaning, but the user risks being labeled as a quaint old baby boomer. However, standard usage never accepted the literal sense of the phrase. Don’t say, “I put my purse down and now I don’t know where it’s at,” unless you want to be regarded as uneducated. “Where it is” will do fine; the “at” is redundant.