I.
Etymology: Middle English, from Old Norse theirra, theira, gen. plural demonstrative & personal pron.; akin to Old Norse that, neuter demonstrative pron. — more at that
obsolete
possessive of they I
II. thər, (|)the](ə)r, (|)tha(a)](ə)r adjective
Etymology: Middle English, from their, pron.
1.
a. : of or belonging to them or themselves as possessors : due to them : inherent in them : associated or connected with them
their furniture
their rights
their neighbors
b. : of or relating to them or themselves as authors, doers, givers, or agents : effected by them : experienced by them as subject : that they are capable of
their verses
their confidence in you
responsible for their being here
doing their utmost
c. : of or relating to them or themselves as object of an action : experienced by them as object
their defeat
their being seen
d. : that they have to do with or are supposed to possess or to have knowledge or a share of or some special interest in
they know their algebra
they like their leisure
e. : that is especially significant for them : that brings them good fortune or prominence — used with day or sometimes with other words indicating a division of time
the twins had a wonderful birthday party; this certainly was their day
2. : his or her : his , her — used with a singular antecedent that is indefinite or that does not specify gender
anyone in their senses — W.H.Auden
we shall be pleased to send a free specimen copy … to a friend or relative on receipt of their address — London Calling
3. obsolete — used after a plural or collective noun or a group of two or more nouns to indicate a possessive case relation
in the father, mother, and governess their absence — The Lives of Women Saints
4. archaic : of those — used especially as antecedent to a relative pronoun
nor better was their lot who fled — Sir Walter Scott