A tack, according to the 6th entry for the word in http://www.dsl.ac.uk/dsl/ , was a lease, usually for a given period; the term could also apply to the property leased. And a tacksman was "one who holds a lease, a lessee, specifically one who leases land, a tenant farmer, or one who leases land to sublet, also a lessee of property, mills, fishings, the collection of customs, teinds, dues, etc. This I am sure is the usage in the Rothisholm connection: it simply meant the individual who held a lease of these lands. At one end of the spectrum a tacksman was thus "just" a tenant farmer, at the other the term was often applied to the lessee of the earldom estate. In the latter case this tacksman paid an annual tak, in exchange for which he was entitled to collect the superior dues and the rents of the sub tenants of earldom lands. After good harvests the tacksman was involved quite a bit of work to collect his entitlements, but a very nice profit could be made. However after a bad harvest the earldom tacksman could be out of pocket, and after a series of bad harvests he could be bankrupted - as was Alexander Brand in the 1690s. James Irvine.