His mother felt he wasmarrying beneathhim (=marrying someone who was not good enough).
I consider such behaviour to bebeneath contempt (=so bad you have no respect for the person involved).
5in a lower, less important rank or job than someone else [= below]
WORD CHOICE: WORD CHOICE: under, underneath, below, beneathUnderis the usual way to say that one thing is at a lower level than another, or is covered by it• Your shoes are under the table.• He lay under a blanket.Underis nearly always used as a preposition (followed by a noun).Underneathhas a similar meaning to under, and is used especially when something is hidden or covered• a box underneath the floorboardsIt can also be used as an adverb• She lifted the cover and peeped underneath.Below is used when one thing is at a much lower level than another, and can be a preposition or an adverb• the apartment below ours • Mist lay in the valley below.Beneathcan be used in the same ways as underand below, but is a more literary or formal word• beneath the silvery moon • the cliff, and the ocean beneath ➔ See alsounder
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.