My first time hearing that word too, but apparently: https://en.wikipedia.org/wiki/Truckle
late Middle English (denoting a wheel or pulley): from Anglo-Norman French trocle, from Latin trochlea ‘sheaf of a pulley’. The current sense dates from the early 19th century and was originally dialect.
Indeed, you will note that they carefully chose the moniker “Daily Active Uniques” and not “Daily Active Users”.
I think that speaks volumes, as humans are definitely harder to retain.