“Thy pyramids built up with newer might / To me are nothing novel, nothing strange”
In Elizabethan English 'pyramids' covered any tall, tapering monument — obelisks, columns, triumphal arches, or spectacular new buildings designed to look ancient — not only the Egyptian structures. The speaker dismisses Time's grandest productions as mere repackagings of older things, not genuine novelties.
“They are but dressings of a former sight”
'Dressings' means costumes or disguises — new clothes put on an old body. The theatrical sense is deliberate: Time is cast as a stage manager who re-dresses the same old props to make them look fresh. 'A former sight' means something already seen before.
“What thou dost foist upon us that is old”
'Foist upon' means to pass off something false or inferior as genuine — to deceive someone into accepting a fraud. Time tricks people into mistaking recycled things for novelties because individual lives are short enough that no one can remember the originals.
“Thy registers and thee I both defy, / Not wondering at the present nor the past, / For thy records and what we see doth lie”
'Registers' are official written records — parish rolls, legal archives, chronicles — that claim to document the truth of events. The speaker defies both Time itself and all its documentary apparatus, because Time's own 'continual haste' distorts what the records report, making them lies.