Jacob's thigh was disabled by the Angel, to show the patriarch that his prevalence was through his faith and prayer, not through force, Genesis 32:25-31. Smiting the thigh was a gesture of self-condemnation and grief, Jeremiah 31:19 Ezekiel 21:12.
In general, a datum is a base elevation used as a reference from which to reckon heights or depths. A tidal datum is a standard elevation defined by a certain phase of the tide.
Common chart datums are lowest astronomical tide and mean lower low water. A chart datum is a vertical datum and must not be confused with the horizontal datum for the chart.