"one who or that which extends or stretches," 1610s, agent noun from extend (v.). Middle English had extendour "surveyor, assessor."
Entries linking to extender
extend v.
early 14c., "to value, assess," from Anglo-French estendre (late 13c.), Old French estendre "stretch out, extend, increase," transitive and intransitive (Modern French étendre), from Latin extendere "stretch out, spread out; increase, enlarge, prolong, continue," from ex "out" (see ex-) + tendere "to stretch," from PIE root *ten- "to stretch."
Original sense in English is obsolete. From late 14c. as "lengthen or extend in time," also "straighten" (an arm, wing. etc.). Meaning "make longer and/or broader in space" is from early 15c., as is intransitive sense of "cover an area, have a certain extent in space;" sense of "expand, grow distended" is from 1753. Related: Extended; extending.