1. Unthankful; ungrateful.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• thankss,
• thanksss,
• thankes,
• thinkness,
• thankls,
• tankliess,
• changless,
• thankled,
• thinkless,
• thanklese,
• thankleses,
• thanklesse,
• thnkless,
• thakless,
• thankles,
• thaknless,
• thankelss,
• thancless,
• chapelless,
• clanklese

The adjective form of the word "thankless" is "unappreciated".

The plural form of the word "thankless" is "thanklesses" However, it is not commonly used and might be more appropriate to rephrase the sentence to avoid using the plural form.

The singular form of the word "thankless" is "thanklessness"