underˈlease,v. [under-1 8 d.] trans. To sublet by an underlease. Also absol. 1819Rees Cycl. XXXV. s.v.Tenure, The takers having the liberty..to under-lease to other tenants.1885Law Times LXXIX. 233/1 A large plot of ground is leased for a term of 999 years. A part of this is underleased for 990 years.