When forty winters shall besiege thy brow,
And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gaz'd on now,
Will be a totter'd weed of small worth held:
Then being asked, where all thy beauty lies,
Where all the treasure of thy lusty days;
To say, within thine own deep sunken eyes,
Were an all-eating shame, and thriftless praise,
How much more praise deserv'd thy beauty's use,
If thou couldst answer 'This fair child of mine
This forms a reply to the question posed two lines above. The poet says the riches of youth are  long gone and now all that is left is hollow eyes, defaced by age. This is also, possibly, an allusion to the effect of “sexual excess” and maybe sexually  transmitted disease — known generically at the time as “the pox”.
Sonnet 2