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.
First, what it is not. It is not a singular form of the word data which some of you may have heard. Data is a collective noun and the plural of datum is datums when used in a geophysical sense. For GPS, navigation, and geophysical use a datum represents a reference point from which you can measure things.