1 [precatio]. Additional services or *boon work required by a lord over and above those which formed a part of tenure. These were ad precem = at the request (of the lord). [< Lat. prex = a request, a prayer]
2) A loan received as a benefit by a tenant from a landowner. No set term or charge was specified. Often, too, the landowner was the patron of the tenant, his protector and lord. The land was held in *usufruct. The closest English word is 'loan'. Like any loan it could be called in; from this that we have 'precarious'.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.