The term 'SOURCE OF A BLAST' can be solved by two four-letter words: PAST and HORN. The word PAST is a noun that refers to a time before the present, or a former state or condition. The word HORN is a noun that refers to a hard growth of keratin that protrudes from the head of certain animals, or an instrument made from a HORN or similar material that produces sound when blown. Together, these two words can refer to a BLAST from the PAST, or a loud sound coming from a HORN.