I can't resist trying to find patterns in words. These three words, for example, seem to fit a similar pattern:
- 頷{うなづ}く = [項]{うな} + 突{つ}く (source: community wiki)
- 躓{つまづ}く = [爪]{つま} + 突{つ}く (source: community wiki)
- 額{ぬか}づく = [額]{ぬか} + 突{つ}く (source: gogen-allguide)
However, I can't seem to find the etymology of 跪{ひざまづ}く anywhere. Since it appears to fit the pattern, I'd like to guess:
- 跪{ひざまづ}く = [膝]{ひざ} + ま + 突{つ}く
But where does the ま come from? It doesn't seem to fit the pattern neatly like the others.