'semi-fixed' originates from a combination of Latin elements: the prefix 'semi-' (from Latin 'semis'/'semi-') meaning 'half', and 'fixed' from Latin 'fixus' (past participle of 'figere') meaning 'to fasten'.

'semi-' entered English via Latin (and via Old French in some compounds) while 'fixed' came into English via Old French 'fixe' from Latin 'fixus'; the compound form 'semi-fixed' developed in modern English by combining these elements.

Initially the elements meant 'half' + 'fastened', and over time the compound evolved to mean 'partly fixed; not completely permanent'.