The baseball player stuffed his bat with cork to make it lighter.
Examples of cork
cork
For festive occasions, pop the cork on a bottle of cava, Spain's sparkling wine.
From the businessweek.com
This one looks like papier-mache, but actually it's a cork bark leaf-tail gecko.
From the bbc.co.uk
Roll the can along the cork, adding another line of glue every couple of inches.
From the kentucky.com
The piece has a hole in one end, which I think was where a cork might be placed.
From the dailyherald.com
The practice of sniffing a wine cork has been pooh-poohed as simply showing off.
From the heraldtribune.com
I came home for my sister's baby shower and left with a cork in my running shoe.
From the denverpost.com
Cork squares are durable, made from natural wood fibers and can handle moisture.
From the courier-journal.com
If you don't want whole 75c bottle at a picnic, share one or stick a cork in it.
From the guardian.co.uk
Cork and Mascarenhas must be wondering what they have to do to pick up a wicket.
From the theargus.co.uk
More examples
Outer bark of the cork oak; used for stoppers for bottles etc.
Close a bottle with a cork
Phellem: (botany) outer tissue of bark; a protective layer of dead cells
Stuff with cork; "The baseball player stuffed his bat with cork to make it lighter"
A port city in southern Ireland
The plug in the mouth of a bottle (especially a wine bottle)
Cork is a rock duo/supergroup consisting of Eric Schenkman (formerly of the Spin Doctors) and Corky Laing (formerly of Mountain). ...
Cork (from corcach, meaning "swamp") is the second largest city in the Republic of Ireland and the island of Ireland's third most populous city. It is the principal city and administrative centre of County Cork and the largest city in the province of Munster. ...
Cork is an impermeable, buoyant material, a prime-subset of generic cork tissue that is harvested for commercial use primarily from Quercus suber (the Cork Oak), which is endemic to southwest Europe and northwest Africa. ...