False yourself. A "dollar" was originally, and in the proverbial grandparent's day, a denotation of an amount of gold. Before fiat money came along, one could exchange gold notes for gold and silver notes for silver, and the amount of gold or silver one exchanged (either way) didn't change with time.
Once the link between currency and any kind of backing store was broken, the "value" of the money was no longer fixed. An hour of labor, a pencil, or whatever, no longer had some common measure (or they did, but the paper money devalued).