A lug is a projecting thing that fits into a slot. The idea is to keep a door closed, or to stop something falling apart.
Battery compartments usually work on this sort of a system, and they are more or less everywhere. Try the back of a clock.
If you're a sea fisherman then you might use a lug (short for lugworm) as bait. These creatures live under the sand doing not very much and are apparently very tasty. If you're a fish.
Signs of lugs: photo from wikipedia uploaded by Nveitch
Here's an actual lug:
photo by M.Buschmann
A lug is also a large basket for fruit or vegetables, or a square sail hoisted on a yard:
illustration from Yosemite~commonswiki
But easiest for most of us to spot is a good Northern British lug, which is an ear (or a stupid man - but this, of course, much more difficult to spot...possibly.). The derived word lug'ole (ear hole) is widely used throughout Britain.
Monday is not a day for verbs, but lug as a verb, meaning to carry effortfully, has given us luggage, which is, rather sweetly, a mixture of baggage and lug.
Spot the Frippet: lug. The verb to lug might be something to do with the Norwegian lugge, which means to pull by the hair (honestly, those Vikings!). The sail might come from the Middle English lugge, pole, or, like most of the other meanings, be to do with the Middle English lugge, meaning ear.
Where the worm sort of a lug got his or her name is, sadly, still a mystery.