What does it mean to say that an allele is lost?

It means it disapear from the population during a certain time period

Explanation:

Lost allele often refers to fixed allele and it is a phenomenon when only one allele of a gene is present in the population. This event occurs usually as a consequence of a disaster, for example forest fire. After the disaster, only certain individuals survive with the fewer alleles that the initial population. Other alleles are lost.

that is wrong, its not thymus.