[ChairmanSaab]

I'm not able to edit my original post so I'll be posting it here. Credits: ChatGPT

There are two ways, both includes editing lookup.js (located at calibre/app/resource/lookup.js or usr/share/calibre/lookup.js in linux)

Option 1. This is the one I use but might break in the future, please replace the entire code of lookup.js with this one.

/* vim:fileencoding=utf-8
 * 
 * Copyright (C) 2019 Kovid Goyal <kovid at kovidgoyal.net>
 *
 * Distributed under terms of the GPLv3 license
 */

(function() {
    "use strict";

    var num_tries = 0;

    // apply the 3px indent to any level-1 define heading
    function indentHeadings() {
        document.querySelectorAll('span[role="heading"][aria-level="1"]')
                .forEach(function(el) {
            el.style.position = 'relative';
            el.style.left     = '3px';
        });
    }

    function fix_google_markup() {
        var cc = document.getElementById('center_col');
        if (!cc) {
            if (++num_tries > 10) return;
            setTimeout(fix_google_markup, 100);
            return;
        }

        // zero-out column gaps everywhere
        var style = document.createElement('style');
        style.textContent = `
           * {
             column-gap: 0px !important;
             -webkit-column-gap: 0px !important;
           }
        `;
        document.head.appendChild(style);

        var max_width = 'calc(100vw - 25px)';
        cc.style.maxWidth   = max_width;
        cc.style.marginLeft = '0';

        var rcnt = document.getElementById('rcnt');
        if (rcnt) rcnt.style.marginLeft = '0';

        var cnt = document.getElementById('cnt');
        if (cnt) cnt.style.paddingTop = '0';

        var s = document.getElementById('search');
        if (s) s.style.maxWidth = max_width;

        var params = new URLSearchParams(location.search.slice(1));
        var q      = params.get('q');

        if (q && q.startsWith('define:')) {
            // inline layout tweaks for define: results
            cc.style.position     = 'absolute';
            cc.style.top          = '0';
            cc.style.left         = '0';
            cc.style.paddingLeft  = '0';
            cc.style.paddingRight = '6px';

            ['sfcnt','top_nav','before-appbar','appbar',
             'searchform','easter-egg','topstuff']
            .forEach(function(id) {
                var e = document.getElementById(id);
                if (e) e.style.display = 'none';
            });

            // wrap definition text
            document.querySelectorAll('[data-topic]')
                    .forEach(e => e.style.maxWidth = max_width);

            // now that the define: block is up, indent headings
            indentHeadings();

            // observe any future DOM changes in center_col and re-indent
            new MutationObserver(indentHeadings)
                .observe(cc, { childList: true, subtree: true });
        }

        // remove the promo sidebar if present
        var promo = document.getElementById('promos');
        if (promo) promo.remove();

        // wrap search results
        document.querySelectorAll('[data-ved]')
                .forEach(e => e.style.maxWidth = max_width);

        // prevent overlap of search results + citations
        document.querySelectorAll('cite').forEach(function(elem) {
            var wrapper = elem.closest('div');
            if (wrapper) wrapper.style.position = 'static';
        });
    }

    if (location.hostname === 'www.google.com') {
        window.addEventListener('DOMContentLoaded', fix_google_markup);
    }
})();

Option 2: Has a minor bug where it hides first letter of the heading, not an issue as the same word is visible below. This won't break with updates, just replace again.

        var cc = document.getElementById('center_col');
         // --- paste this code below the above line, ctrl+f in your fav text editor to find the line ---
         var style = document.createElement('style');
         style.textContent = `
           * {
             column-gap: 0px !important;
             -webkit-column-gap: 3px !important;
           }
         `;
         document.head.appendChild(style);
         // --- and the between the below line, you can leave this comment if you'd like ---
        var max_width = 'calc(100vw - 25px)';