It is 100 years since R A Fischer introduced the concept of "variance" (in his 1918 paper The Correlation Between Relatives on the Supposition of Mendelian Inheritance).
"Bag" refers to "bootstrap aggregating," repeatedly drawing of bootstrap samples from a dataset and aggregating the results of statistical models applied to the bootstrap samples. (A bootstrap sample is a resample drawn with replacement.)
I used the term in my message about bagging and several people asked for a review of the bootstrap. Put simply, to bootstrap a dataset is to draw a resample from the data, randomly and with replacement.
The field of data science is rife with terminology anomalies, arising from the fact that the field comes from multiple disciplines.