Where does “extensively” come from?

Extensively derives from Middle English -ly added to extensive, which comes from Latin extensivus, formed from the Latin prefix ex- and a suffix ultimately from Proto-Indo-European -iHwós.

extensively (English): In an extensive manner, widely; To a great extent

Definitions

In an extensive manner, widely; To a great extent

Ancestry of “extensively”, step by step

Step	Language	Word	Meaning
1	English	extensive	having a great extent; covering a large area;...
2	Late Latin	extensivus	extensive, extendible
3	Latin	extēnsus	—
4	Latin	extendō	to stretch or thrust out
5	Latin	tendō	to stretch, stretch out, distend, extend
6	Ancient Greek	τένων	tendon, sinew
7	Proto-Indo-European	ten-	to stretch, to extend

Every word from Proto-Indo-European ten- →Every word from Ancient Greek τένων →Every word from Latin tendō →