what's a pitiful liar?

It means you are not good at lying.

Roger is correct. "Pitiful" in this case means "poor." It means you're a very poor liar, that is not good at lying.

thank you Roger

thanks a lot

then what about "What reasoning could have prompted such advice?" this sentence?

There is a trap in the usage you describe, and what you really want to say is "I am a POOR liar". That actually means that you are bad at lying, and hence rarely or never lie.

Saying somebody is a "pitiful liar" means that he or she is a HABITUAL liar and that the situation is pitiful.

Similarly calling somebody a "sorry asshole" doesn't means that he's sorry about being an asshole, it means he's an asshole and the situation (of him being an asshole) is sorry.

Thank you for you reply, but could you also help me with "What reasoning could have prompted such advice?" this sentence? I'm not quite sure about what this one means here. Thank you

This all goes back to the notion that if a woman hasn't got married by 30 she's a failure. That's why there's the need to lie. You don't get the same attitude towards unmarried men, because they could be involved in other things, and marry later in life, but for the woman there was only domesticity.

This notion is dated, but it still clings on in the mindset of a lot of people. There's a children's card game called Old Maid there are lots of pairs, and the object is to pair up all the sets, but there's only one Old Maid card, and whoever is left with that is the loser.

This attitude was espoused by countless Doris Day films, which have been brilliantly parodied by French & Saunders.