single sword; one sword
Refers to having or using only one sword, often in contrast to dual-wielding. Can appear in historical or martial contexts.
He faced the enemy with only one sword.
wielding a single sword; direct and to the point
The second meaning is almost exclusively used in the set phrase 単刀直入 (たんとうちょくにゅう), meaning 'cutting straight to the point'. The standalone use of 単刀 for 'being straightforward' is rare and mostly literary or idiomatic.
See also: 単刀直入
The four-character idiom 単刀直入 means getting straight to the point without preamble.
straight to the point; point-blank; frank; direct
Compound of 単 (single) and 刀 (sword). The extended meaning of directness comes from the image of charging in with a single sword, as seen in the idiom 単刀直入.