The expression "Tank it" derives its origin from the term "tank" in slang usage, where it commonly designates a person or a player who is able to take or handle the brunt of something (such as alcohol, aggressions, enemy fire, enemy attacks, difficult situations, criticism, reprehensions, etc.). It is therefore used as an encouragement similar to "You can do it", "Keep it up", "Go all out", "Stand strong", "Pull through" or even "Good luck" and "Break a leg"; while implying that the person directed should be strong and be able to take care of the situation no matter what the odds against them may seem to be.
WARNING: Should not be confused with the more standard verb "tank", meaning "to fail", as in "I tanked my exam" (i.e. "I failed my exam; I did poorly on my exam"). Though this verb is rarely employed in the imperative form, speakers unfamiliar with its secondary slang meaning may interpret it the wrong way.
"Dude, I don't know if I can do this, there are too many enemies"
"You'll be fine, just tank it!"
"I'm getting really nervous, I don't think I can do my presentation"
"Stop thinking so much about it. When the teacher calls you up, get up there and tank it. Do it like a boss, forget about the others."