The narrator goes out of his way to mention that the jawbone Samson picked up was fresh.
So he is touching a freshly killed donkey. This is not so much to let us know that Samson picked a jawbone that would still be heavy and touch, with the teeth likely still fixed in place (as opposed to an old jawbone that has lain for a time in the sun and become dried out and brittle). We are given this detail to see that once again, Samson was violating his Nazirite vow; a Nazirite was not supposed to touch anything dead, whether animal or human. (See note at Judg 14:9 for more.)
But again, we see the Lord using a sinful instrument to achieve victory.
15 And he found a fresh jawbone of a donkey, and put out his hand and took it, and with it he struck 1,000 men.