Underneath means directly below and covered by something. Underneath the bed, underneath the surface, look underneath.
It is stronger and more physical than under: underneath suggests fully covered or hidden, which is why you find things underneath rather than under them. It works as a preposition and as an adverb — have a look underneath — and occasionally as a noun: the underneath of the car. Figuratively it means beneath the outward appearance: underneath it all he was terrified. Note that beneath is the formal equivalent and below the neutral one.
1prepositionunderneathDirectly below and covered by.
“The key is underneath the second flowerpot.”
2adverbplaceIn the place below.
“Lift it up and have a look underneath.”
Too few real examples of this word to estimate which sense is commonest.
underneath is a position below.
Old English underneothan is under plus neothan, meaning below — two words for the same idea stacked on each other. English does this often when a word starts to feel weak, which is how underneath ended up more emphatic than under.
Proto-Germanic
*underniþer
Old English
underneoþan — underneath
Middle English
undernethe
English, today
underneath
From Middle English undernethe, undernethen, from Old English underneoþan (“underneath”), ultimately from Proto-Germanic underniþer. Equivalent to under- + neath.
Etymology from Wiktionary, CC BY-SA