It does not mean "to declare, assert, maintain," etc.

It is nearly the equivalent of _guess_ in the Northern and Middle States, and of _reckon_ in the South.
It agrees precisely with the New England _calk'late_.

Like all the rest of these words it may have a strong sense by irony.

When a man says, "I 'low that is a purty peart sort of a hoss," he understates for the sake of emphasis.

It is rarely or never _allow_, but simply _'low_.