The word "tip" has multiple meanings, so let's look at the antonyms for each of its main senses.
Meaning: the pointed or narrow end of something (like the tip of a pencil)
Antonyms for this sense of "tip" could be "base" or "bottom". The tip is the end that is usually the thinnest or most pointed, while the base is the broader, more stable part that supports the object. For example, if you think of a cone, the tip is at the very top where it narrows down to a point, and the base is the wide circular part at the other end. Similarly, with a pencil, the graphite tip is where you write, and the wooden base is the part you hold.
Meaning: a small amount of money given as a gratuity (a tip to a waiter, for example)
For this meaning, antonyms might include "charge" or "fee". When you give a tip, it's an extra amount you choose to give voluntarily on top of the regular cost of a service. In contrast, a charge or fee is something that is required to be paid as part of the normal cost of using a service or getting a product. For instance, if you go to a restaurant, you pay the bill for your meal (which includes the cost of the food and any applicable taxes), and then you might optionally give a tip to the waiter. But if there was an additional cover charge or a service fee that was mandatory, that would be the opposite of a tip in the sense that it's not a voluntary extra but a required payment.
Meaning: a piece of useful advice or a hint (a tip on how to solve a problem)
Antonyms for this sense could be "confusion" or "misinformation". A tip is something that helps you understand or do something better. It gives you clear direction or useful knowledge. On the other hand, confusion makes things less clear and harder to understand, and misinformation is incorrect information that can lead you astray instead of helping you like a tip would. For example, if someone gives you a tip on how to fix a leaky faucet by tightening a certain valve, that helps you. But if you get misinformation that tells you to do something that will actually make the leak worse, or if you're just left in confusion not knowing what to do, that's the opposite of a useful tip.
So, depending on which meaning of "tip" you're considering, there are different sets of antonyms that apply.