Subcutaneous means under the skin. It is a medical word, not an everyday one — doctors and nurses use it, and you meet it printed on medicine boxes and in hospital notes. A subcutaneous injection goes into the soft fatty layer just beneath the skin, not deep into a muscle and not into a blood vessel; insulin is the famous example, which is why people with diabetes learn this word early. You will also see 'subcutaneous fat' (the fat you can pinch, sitting between skin and muscle), 'subcutaneous tissue', and 'a subcutaneous lump'. In speech, medical staff nearly always shorten it: they say 'sub-cut' or write 'SC' or 'subQ' on a chart. If you hear a nurse say 'give it sub-cut', that is this word. It says nothing about being hidden or secret in a feelings sense — for that you want 'subconscious'. This one is purely about a physical layer of the body, and it works the same way for animals: a vet gives a cat subcutaneous fluids.