Pensieve seems to be a portmanteau, combining the words ‘pensive’ and ‘sieve.’ The latter is an object in which something may be sorted, and ‘Pensive’ is derived from French, and originally from the Latin ‘pensare,’ meaning ‘to ponder,’ and in common English usage means ‘thoughtful’ or ‘reflective;’ thus a ‘pensieve’ allows for the sorting of thoughts, or memories.