Meaning

noun a thin crisp wafer made of flour and water with or without leavening and shortening; unsweetened or semisweet

noun a poor White person in the southern United States

noun a programmer who cracks (gains unauthorized access to) computers, typically to do malicious things

originFrom cracker + -s. The South African sense is from their sound and its status as a plurale tantum by association with trousers. The adjectival sense derives from British naval expressions referring to firecrackers in one's head, originally as "he's got the crackers" and then "he's gone crackers" before the present "he is crackers".

Etymology from Wiktionary, CC BY-SA 3.0