The adjective 'thinkable' is a modern English formation, combining the native Germanic verb 'think' with the productive Latinate suffix '-able'. The core verb 'think' originates from the Proto-Germanic þankijaną, related to concepts of thought and thanks, and has retained its central meaning of mental conception. The suffix '-able', entering English via Old French from Latin -abilis, systematically converts verbs into adjectives denoting possibility or capacity. Thus, the literal construction "capable of being thought" logically yields the modern abstract sense of something that is conceivable or within the realm of imagination.

Structure: think (to form ideas) + able (capable of) - think: From Old English þencan (to conceive in the mind, consider), of Germanic origin. - able: From Latin -abilis, a suffix forming adjectives meaning "capable of, fit for, tending to".