From Longman Dictionary of Contemporary Englishheadsheadsthe side of a coin that has a picture of a person’s head on itheads or tails? British English spoken (=used to decide something, by asking someone which side of a coin they guess will be showing when you throw it in the air and it lands) → tails at tail1 → head
Examples from the Corpus
heads or tails?• Sometimes even the lawyers can not make heads or tails of the instructions.• Technicians can identify only obvious problems, such as malformed sperm heads or tails. 