'unanchorable' is formed in modern English by adding the negative prefix 'un-' (not) and the adjectival suffix '-able' (capable of) to the noun 'anchor' (from Old English 'ancor', via Latin 'ancora', ultimately from Greek 'ankura' meaning 'hook').

'anchor' changed from Old English 'ancor' (borrowed via Latin 'ancora') and ultimately from Greek 'ankura'; the modern English noun 'anchor' then takes productive affixes 'un-' + '-able' to form 'unanchorable'.

Initially, the root 'anchor' meant 'hook' and later 'a device for mooring'; the derived adjective 'unanchorable' simply means 'not able to be anchored' and has retained that literal sense (with occasional figurative extensions).