Definition from Wiktionary, the free dictionary
From Swedish planka.
- A plank (long, broad and thick piece of sawn timber).
Declension of lankku (type valo)
Usage notes 
- There's no one exact definition, but generally sawn timber must be at least 38 mm (1,5") thick and 75 mm (3") wide to be called lankku. If the cross-section is close to square, heavy timber will be called parru. A heavy plank may also be called palkki.