What does 'On the nod' mean?

Meaning 1:

If something is accepted by parliament or a committee majority, it is on the nod.

Meaning 2:

Someone who's on the nod is either asleep or falling asleep, especially when the shouldn't or are are in a position unusual for sleep, like sitting or standing.

Meaning 3:

When a horse runs, its head moves backwards and forwards alternately - in horse racing, if 2 horses cross the line together the one whose head happens to be going forward often wins and is said to win 'on the nod'.

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