c. 1300, fysike, phisike, "a healing potion;" early 14c., "natural science;" mid-14c. "healthful regimen;" late 14c., "the art of healing, medical science or theory;" from Old French fisike "natural science, art of healing" (12c.) and directly from Latin physica (fem. singular of physicus) "study of nature," from Greek physikē (epistēmē) "(knowledge) of nature," from fem. of physikos "pertaining to nature," from physis "nature," from phyein "to bring forth, produce, make to grow" (related to phyton "growth, plant," phylē "tribe, race," phyma "a growth, tumor") from PIE root *bheue- "to be, exist, grow."
The English spelling with ph- is attested from late 14c. (see ph). The meaning "medicine that acts as a laxative" is from 1610s. The obsolete verb meaning "to dose with medicine, administer medical treatment to" is attested from late 14c. (phisiken).
Physics, in Aristotle, is the science of what the Greeks called 'phusis' (or 'physis'), a word which is translated "nature", but has not exactly the meaning which we attach to that word. ... 'Phusis' has to do with growth; one might say it is the 'nature' of an acorn to grow into an oak, and in that case one would be using the word in the Aristotelian sense. The 'nature' of a thing, Aristotle says, is its end, that for the sake of which it exists. [Bertrand Russell, "History of Western Philosophy"]