Buckle up is a phrase that is used to remind people to FASTEN their seatbelts when they are in a car. The crossword solutions for this term are BUTTERCUP (9 letters), FASTEN (6 letters), WARP (4 letters), and Strapin (7 letters). BUTTERCUP is a type of flower, while FASTEN is an action verb meaning to secure something in place. WARP is a verb meaning to twist or bend out of shape, and Strapin is a combination of the words strap and in, referring to the act of putting on a seatbelt.