Always paired with 打 (to hit / make): 打呵欠 = 'to yawn'. Variant 哈欠 is more common in northern speech and dictionaries; 呵欠 is the older listed form. Both 打哈欠 and 打呵欠 are correct.
memory
欠 alone originally meant 'to yawn' - its oracle-bone form pictures a person with mouth open, exhaling. The modern noun adds 呵 (the breath sound) to make it explicit.
