[Joe Brower]

"I think it means 'cover yer six'."

Thanks; I think that's it, too. I guess Squantos ain't gonna tell me -- it's a trade secret. $;-)

[JackFromTexas]

From the fur trade days, and then abouts, 'topknot' is your scalp - the phrase means watch out for those who would scalp you . . .
Cover your six is fighter pilot term for similar situations - don't let the bad guys get behind you / in your blind or weak spot.