Defer “to put off, delay” comes from Middle English deferen, differren “to put off, delay, be different, differentiate, refer a matter for decision; defer to, show respect or deference to,” from Old French def(f)erer, dif(f)erer “to have different qualities, be different, be dissimilar,” from Latin differre “to carry away, carry in different directions, differ, postpone, adjourn.” Differre is composed of the prefix dif- (a variant of dis- used before f ) “apart, asunder” and the simple verb ferre “to carry, bear.”
Defer and differ were originally the same word, but spellings with def- for etymologically correct dif-, which first appeared in the 15th century, have become standard in part because of the sense “to put off, delay” (absent in differ ), in part because of the accent being on the root (second) syllable, and in part through association with delay .
The meaning “to exempt temporarily from military service” first appeared in 1941.

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