Why was Andrew Jackson nicknamed 'Old Hickory'?

Andrew Jackson was 'Old Hickory'

He was named so because he was said to be "as tough as old hickory" - a very tough wood.

How'd he get the name?

His TN militia men gave him this name after he led them back from New Orleans, walking most of the way, since he let the wounded soldiers ride his horses.

Hickory is a very tough kind of wood and the allusion was to his toughness.