stiff arm [stif ahrm]
verb.

1. When a tandem nursing twin baby places his or her inward facing arm such that the elbow is not bent and then pushes away his or her sibling forcefully.

Warning, a stiff arm may result in the following:

• crying
•  spilled milk
• scratched faces
• uncontrollable laughter
• smiles
• distracted babies

Nathan has gotten to where he will hold Anthony’s arm down by his wrist so Anthony can’t stiff arm him while they eat together.