Acronym: "Free Of Charge, No Strings Attached". Refers to a service that is provided for free and on no explicit conditions, typically in an interpersonal context.
When I was bedridden recovering from my abdominal surgery, my neighbors Natalie and her husband Jacob offered to help handle house chores and cook meals for me. It was all offered FOC/NSA too! I'm glad there's still some kindness left in this world.