XY refers to a boy (male).
In humans, sex is determined by a pair of chromosomes known as sex chromosomes. Females typically have two X chromosomes (XX), while males typically have one X chromosome and one Y chromosome (XY).
Sex Chromosomes and Determination
The presence of the Y chromosome is what determines maleness in humans. The Y chromosome contains the SRY gene (Sex-determining Region Y gene), which triggers the development of the testes. If the SRY gene is present (i.e., XY chromosome combination), the embryo will develop into a male. If the SRY gene is absent (i.e., XX chromosome combination), the embryo will develop into a female.
Summary
Therefore, the XY chromosome combination indicates that an individual is biologically male, and thus corresponds to "boy".
