Heads-up and Shotgun

Heads-up noun - The act or an instance of telling beforehand of danger or risk.

Shotgun and heads-up are semantically related. In some cases you can use "Shotgun" instead a verb phrase "Heads-up".

Shotgun noun - Firearm that is a double-barreled smoothbore shoulder weapon for firing shot at short ranges.

Heads-up and shotgun are semantically related. Sometimes you can replace phrase "Heads-up" with "Shotgun", this terms are similar.