feeble lie

When something is said and it is very unlikely that it is a lie (yet not impossible) but due to lack of witnesses, and possible verification, it cannot be proven wrong, and legally is a lie

Jeff: I heard you go out with that ugly fat girl Helga

Paul: Yeah, right I do not go with her

Jeff: She said you guys are dating

Paul: Well she's wrong

Jeff: That's a feeble lie man, your definitely lying to me but I can't prove it