can you please tell me if this is a compound noun or a compound adjectif:
turtle necked jumper.
and please tell me why.
'Turtle necked jumper' is a noun phrase. The first two words should be hyphenated thus: 'turtle-necked' because they function as a compound adjective ending in 'ed'. 'Jumper' is a noun modified by the compound adjective 'turtle-necked'. A compound noun is usually two or more nouns used together, one of them being modified by others; for example, 'safety net', language expert, etc.