by Alex Beregszaszi

Are you new to Github and/or contributing to open source projects? Have you seen short messages like LGTM, ACK, NACK, etc. and wondered what they mean?

Here you go:

  • LGTM — looks good to me
  • ACK — acknowledgement, i.e. agreed/accepted change
  • NACK/NAK —negative acknowledgement, i.e. disagree with change and/or concept
  • RFC — request for comments, i.e. I think this is a good idea, lets discuss
  • WIP — work in progress, do not merge yet
  • AFAIK/AFAICT — as far as I know / can tell
  • IIRC — if I recall correctly
  • IANAL — “I am not a lawyer”, but I smell licensing issues

Many projects in the crypto space also use the following (popularised by Bitcoin’s hacker lingo):

  • Concept ACK — agree with the concept, but haven’t reviewed the changes
  • utACK (aka. Untested ACK) — agree with the changes and reviewed them, but didn’t test
  • Tested ACK — agree with the changes, reviewed and tested

These answers are usually part of the code review process and you would find them in issues or pull requests in Github.

Honorary mention: +1 as the short form of ACK (and in many cases, Concept ACK). After the famous “Dear Github” letter, the platform has introduced proper reactions to declutter comments. No, this is not about making Github your next Facebook :)

You would also see the ACKs be included in commit messages, like how the Linux kernel does it since Git is being used:

Add get_random_long().Signed-off-by: Daniel Cashman <>Acked-by: Kees Cook <>Cc: "Theodore Ts'o" <>Cc: Arnd Bergmann <>Cc: Greg Kroah-Hartman <>Cc: Catalin Marinas <>Cc: Will Deacon <>Cc: Ralf Baechle <>Cc: Benjamin Herrenschmidt <>Cc: Paul Mackerras <>Cc: Michael Ellerman <>Cc: David S. Miller <>Cc: Thomas Gleixner <>Cc: Ingo Molnar <>Cc: H. Peter Anvin <>Cc: Al Viro <>Cc: Nick Kralevich <>Cc: Jeff Vander Stoep <>Cc: Mark Salyzyn <>Signed-off-by: Andrew Morton <>Signed-off-by: Linus Torvalds <>

Check out the “How to Get Your Change Into the Linux Kernel” guide for a thorough explanation.

Similar short answers are used widely in software engineering and the open source community as they make communication more efficient.

You have surely seen the following in source code — TODO, FIXME, XXX and NOTE — and only wondered what XXX means?

Interested to see a lot more acronyms with explanation and perhaps a bit of history? Check out The Jargon File. It is the definitive source since 1975.

Bonus trivia: where does ACK/NACK comes from?

I would say it came from networking/interface protocols, perhaps the popularity of TCP caused widespread usage.