To combine something into a liquid, substance, or mixture by stirring. A noun or pronoun can be used between "stir" and "in." You need to stir the oil in very slowly so that it mixes properly with the egg yolks.She stirred in a bit of honey to make the medicine taste a little better.
stir in
v.
To introduce something, such as an ingredient, into a liquid or mixture while stirring: The fruit punch tasted a bit bland, so I stirred in a cup of grape juice. Once the sauce is simmering, stir some parsley in.