Jacob's commentary was meaningless as he warned her against her action, because Janet had already decided to buy the lottery ticket from the mini-mart.
Assuming you mean "how" and not "why" ( since that is unanswerable) a grammatical sentence may be meaningless, it is possible a number of ways. It may be contain nonsense words like "runcible" or...
When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).