What's in a "Lap"?

I was lying in bed this morning and for some reason I started thinking about what a "lap" is. You know, like when someone sits on someone's "lap". If you really think about it, it's weird. A lap is really just your thighs, right? I think it's the only part of the body that we call a different name depending on what it's doing. If you're just standing up it's not a lap is it? It's just your legs or your thighs, but if you're sitting down, all the sudden it turns into a lap. Who even came up with this? I'm so confused....

Dictionary.com defines a lap as: the front part of the human body from the waist to the knees when in a sitting position. When in a sitting position?! Why don't we call our arms something else when we're sitting? I think I'm going to start making up my own words for similar situations. For example, when you sit in a cross-legged position, your legs are no longer "legs". They will now be referred to as "franks". When your tongue is in the resting position in your mouth while not talking, your tongue will be known as a "domino".

Weird.