Contraction of Last and Bastard. Used to describe the person who leaves the empty roll on the paper holder, empty milk bag in the container etc. Lastards also do things like cramming one last thing in the trash rather than taking it out. In some families it also refers to the person who takes the last slice, chip, cookie etc when not everyone has had a first.
Who's the dirty lastard who took the last ice-cube without adding some water to the tray?!