A Grakk is a sometimes used term for a costly, painful or embarassing error made in game or in real life,
Sending off a fleet save and going to bed only to find out you left all your res on the moon. A Grakk.
Falling asleep and losing a friends fleet you are supposed to vmode. A Grakk.
Sending rips to destroy a mate's gravitron sats (before scrap merchant option was available) and wiping the moon defenses out, instead of targeting the planet. is a Grakk.
Not every error is a Grakk. The error needs to, when told to friends, elicit a laugh, cringe, shock or anger. The only exception to this minimum rule is once you become known for your Grakks. In this case, an eye-roll or head shake can be enough to indicate a Grakk has occured.