verb (used without object)

(of a whale) to slap the flukes against the surface of the water.

Origin of lobtail

1865–70 (as gerund); apparently lob1 in sense “to move heavily” + tail1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for lobtailing

Historical Examples of lobtailing