When I wore one on Holy Thursday a server asked me what it was, I told him, he misheard and said, "Oh a manacle". In a way Joe was right.
The origin of the vestment seems to be related to the ancient magistrates nappa, a sign of office, but still a towel or napkin, stylised yes. It seems to be more ancient than the stole. Sub-deacons were vested with it at their ordination, the prayer that accompanies vesting with it speaks of labour, drudgery even:
Grant, O Lord, that I may bear the maniple of weeping and sorrow, so that I may receive the reward for my labours with rejoicing
Its connection with the subdiaconate (abolished by the Council) would suggest it was a vestment denoting servitude. The modern secular equivalent would be the waiters cloth. In France, at one time in the best restaurants every junior waiter wore a white one over his left arm. It should be seen as the foot washing slave's towel. In terms of John's Gospel therefore it is, as I have said before on this blog, it is the sign of "the one who came to serve, not to be served".