An expression of annoyance and dismissal of someone or something. It means the same as "Forget it!" Well, you can lump it because I'm not staying late to help you—that project is your responsibility!
Inf. Forget it!; Go away! Well, you can just lump it! Lump it! Drop dead!
exclam. Forget it!; Go away! (see also Like it or lump it!.) Well, you can just lump it!