What is the antonym of “veering” in the nautical sense?

The anticlockwise counterpart of the verb veer is, prosaically enough, back.

back verb (used without object) ... 30. Nautical. (of wind) to change direction counterclockwise (opposed to veer ).

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However, Collins adds a caveat concerning deixis:

back v 37. (Physical Geography) (intr) (of the wind) to change direction in an anticlockwise direction in the northern hemisphere and a clockwise direction in the southern.

From an ITV weather / shipping forecast:

Wind: Westerly fresh F5 to strong F7, locally southerly or variable light F2 to moderate F4 in Alderney, becoming northerly moderate F4 to fresh F5 by noon, backing northwest in the afternoon, backing southwest to south light F3 to moderate ...