What does it mean to “lack substance “?

Yes. your gf is correct the words "lack substance" means that the person to whom it may be said is probably discussing something and his discussion was off the topic, or was a statement that was less well thought out, or it was too simplistic.

Then someone else says about that person something like this, "His comment lacked substance." The term is antiquated but is a little more sophisticated than "he is stupid." Good luck.

No, she's wrong. Substance is about how much personality and character, depth of feel and emotion, thinking of others instead of just themselves - it's about the character of a person. Having a lack of substance means that a person is shallow - doesn't think about much but perhaps themselves. Kind of like, oh, the Kardashians, someone who cares about their appearance, their public persona and not much else.

Having substance means being solid, full, substantial quality. Here's another example. Lindsay Lohan is flighty and irresponsible, been arrested multiple times, makes excuses for her erratic behavior and has become unreliable and unbankable as an actress. Sandra Bullock on the other hand, is strong, confident, has an exceptional work ethic, and cares for others - she has donated one million dollars each to rescue efforts from the tsunami in Indonesia that killed over 200,000 people, and to Haiti rescue after the devastating earthquake. And just two weeks after the biggest night of her life, she stood strong during a devastating public scandal when it came to light about her husband's cheating scandal. She handled it with strength and dignity.

Depends on the context and what you think substance means.

Your argument lacks substance - suggests though there is no clear meaning, that the argument doesn't have factual information, or isn't persuasive or weighty.

You lack substance - could mean you lack integrity. It could mean you lack intelligence or well formed thoughts. If you interpret substance to be brains.

Substance is a very vague word. Almost by definition!

No, it means you have no personal integrity, no good and redeeming qualities. You are shallow, vapid, even uncaring, no character. It's pretty bad in other words. It has nothing to do with intelligence. You win this one!