Literally: simple/single – sword – direct/frankness/repair – enter
Alternately: Charging into a mass of enemies with just one sword to protect you. Getting straight to the point. Being direct instead of roundabout. Saying something point-blank without niceties or politeness.
Note: There is a tension, in studying foreign culture, between essentializing and Otherizing them (“all Japanese social interactions are ambivalent; the Japanese are trained from birth to be inscrutable and indirect, to place politeness and social self-defense over all else”) and pretending that they are only a trivial variation on a theme (“everybody’s the same no matter where you go, right?”). In my mind this phrase highlights that tension: Japanese people can be direct instead of tactful or considerate when they want… but perhaps there is more danger felt in the act than in the relatively blunt and forward USA.