UK, England, Middlesex—Parish Registers, 1539–1988 [Part A][MSF3-268] Image Name004144231_00135Batch IDMSF3-268
The letters are 'ux' which stands for 'uxor' the Latin for 'wife'.
By the way the child should not be given the surname in this batch unless it is written as part of their name (as per the General Indexing Guidelines under the title Surnames). The Field Help has a confusing example but the Project Instructions have an example 'How to index a Christening Record' that shows the correct way.
Just for general reference to Latin words - the FamilySearch wiki has a Latin word/abbreviation list: