The word 'thru' is a 19th-century American English phonetic spelling reform of 'through,' intended to simplify spelling. 'Through' itself originates from Old English 'þurh,' meaning "from one side to the other," which is related to Old High German 'duruh' and Gothic 'þairh.' These Germanic forms derive from the Proto-Indo-European root terə-, meaning "to cross over, pass through, overcome," which also gives us words like 'trans-' and 'thorough.' The spelling 'thru' represents a direct attempt to align the written form with its modern pronunciation, bypassing the historical orthographic complexities of its parent word.

This is a simplified spelling variant of the word "through." It is not composed of separable morphemes in its modern form.