COMB in ADJ （与knee，shoulder等词组合）表示“到…高度”，“高至…” -high combines with words such as 'knee' or 'shoulder' to indicate that someone or something reaches as high as the point that is mentioned. （与knee，shoulder等词组合）表示“到…高度”，“高至…”
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The grass was knee-high.
草有齐膝高。
-high is also a combining form.
（亦可用于构词）