Whole-body hydration (WBH) assessment is crucial in reducing the risk of dehydration, which can have significant health impacts when left untreated. Existing methods for monitoring WBH are invasive or require bulky equipment, making them impractical for everyday or continuous use. In this work, we introduce an arm-wearable electronic tattoo (e-tattoo) utilizing bioimpedance (Bio-Z) sensing for noninvasive, continuous, and mobile WBH monitoring. Although whole-body Bio-Z is a proven method for indicating WBH status, the effectiveness of using arm Bio-Z for this purpose has been unclear. Our IRB-approved study, which involved diuretic-induced dehydration, demonstrated a strong positive linear correlation between cross-arm Bio-Z and percent body weight loss, with Pearson's r = 0.967 ± 0.031 achieved in six human participants. Furthermore, results indicated that arm Bio-Z outperformed traditional whole-body Bio-Z recordings, likely due to the stretchable, flexible, and bodyconformal nature of the proposed e-tattoo. These findings suggest that the upper arm Bio-Z can serve as a reliable, continuous proxy for WBH, offering a cost-effective and highly accessible solution. The potential uses of this wearable technology range from improving personal wellness to helping professional sports, healthcare, and occupational safety.