'First Strike' was an original ability, created with the beginning of Magic.

702.7. First Strike

• 702.7a. First strike is a static ability that modifies the rules for the combat damage step. (See rule 510, "Combat Damage Step.")
• 702.7b. If at least one attacking or blocking creature has first strike or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that didn't have first strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.
• 702.7c. Adding or removing first strike any time after combat damage has been put on the stack in the first combat damage step won't prevent a creature from dealing combat damage or allow it to deal combat damage twice.
• 702.7d. Multiple instances of first strike on the same creature are redundant.

Translation:

• 702.7a. First strike is always active, and by its presence the combat damage step expands to allow it to act first.
• 702.7b. If some creature in the fight has first strike or double strike, it deals damage before the other creatures deal their normal damage.
• 702.7c. Removing First Strike doesn't allow you to deal normal damage or avoid dealing damage.
• 702.7d. You don't deal damage multiple times for multiple instances of first strike or double strike.