The song actually grew out of Meghan Trainor's frustrations with her record label, rather than an unfaithful lover. Kevin Kadish recalled to Bart Herbison of Nashville Songwriters Association International
they'd actually finished the Title
album but had an extra day together, and Trainor's A&R guy encouraged them to keep writing. Kadish put on a track that he had already started for the record while Trainor was on a conference call. She could hear it playing through the headphone box and the singer banged on the glass, and told the producer, "That's what I want to write today."
Kadish knew the whole situation of what was going on with Trainor and her record label, so while she was on her conference call the producer sat at his computer writing, "I know you're lying, your lips are moving." By the time she walked back into the room he had a bunch of lyrics written down. She loved what Kadish had done and started singing the verse. According to Kadish this all happened in eight minutes.