Fallacious

/fəˈleɪ.ʃəs/ adjective medium

Based on a mistaken belief or flawed reasoning; logically unsound.

The lawyer pointed out that the witness's entire argument rested on a fallacious assumption about the timeline.

Memory Trick

Fallacious shares a root with 'fallacy' and 'fail' — a fallacious argument is one that ultimately fails the test of sound logic.