'Swain' in 'Northswain' is Middle English and has several meanings. It can be 'young man', 'servant', 'shepherd' or 'male lover'. There are two abilities with almost the exact same name. The other is Northswain's Strike.
This ability is 北斗骨砕斬 [hokuto kotsusaizan] or 'Big Dipper bone breaking cut' in Japanese. The Big Dipper is an asterism in the northern skies, formed out of seven stars. It can be found in the constellation of Ursa Major, and is formed out of the stars Dubhe, Merak, Phecda, Megrez, Alioth, Mizar and Alkaid.
Airborne Brigade

R+
Type: STR, Element: -, Growth: Slow, Cost: 16
Attack: 1,080 (lv1), ? (lv80), 6,214 (lv100), ? (lv120), ? (lv140), ? (lv160)
Defense: 1,070 (lv1), ? (lv80), 5,747 (lv100), ? (lv120), ? (lv140), ? (lv160)
Obtain: Friend Recruit campaign (Ashe)
R
Type: STR, Element: -, Growth: Normal, Cost: 12
Attack: 730 (lv1), 2,673 (lv60), ? (lv75), ? (lv90), ? (lv105), ? (lv120)
Defense: 720 (lv1), 2,380 (lv60), 2,803 (lv75), 3,225 (lv90), 3,647 (lv105), 4,070 (lv120)
Obtain: Friend Recruit campaign (Ashe)
Record Keeper

Type: Soul Break, Rarity: -
Target: all, Element: -, Gauge cost: 1 bar
Learn: Ashe (equip Runeblade (XII))
Effect: Attacks all enemies with a non-elemental magic attack and heals all allies for a little
Category: Ability