The word "furthermore" is used to add information or detail to what has already been said. It indicates that additional, often reinforcing, points or arguments are being introduced. Essentially, it helps in building upon a previous statement to emphasize or elaborate on it.
The word "furthermore" is a combination of "further," which derives from Old English "furthera," and "more," which comes from Old English "māra." The suffix "-more" reinforces the idea of additional or supplementary information, leading to its current use in modern English.